1. Field of the Invention
This invention generally relates to a lens system for use in a copier, and, in particular, to a large aperture lens system for use in a copier, having a symmetrical arrangement with respect to a stop.
2. Description of the Prior Art
Recently, demands for high-speed operation and reproduction of high quality images for copiers are rising in addition to compactness in the overall size. As a result, a lens system of a copier is required to satisfy new demands. That is, for a high-speed reproduction operation, a light or bright lens system is required; however, if a lens system is designed to be lighter, its outer diameter becomes larger, which hinders to make the overall size of a copier compact in size. In this case, however, the disadvantages of the lens outer diameter becoming larger may be cancelled to some extent by making the total length of a lens system shorter. Regarding the demand for a high quality image reproduction, although it also depends on the image forming characteristic of a lens system and also on the developing characteristic, a recent trend in an increase in making copies from copies and also in an increase in the kinds of originals, e.g., printed matter, hand-written matter and pictures, must also be taken into account. Under the circumstances, it is necessary to provide a lens system capable of maintaining high MTF from its optical axis to its surrounding and having a large depth of field. In addition, such a lens system is required to have a high contrast from a low frequency to a high frequency.
In general, for use in copiers, printers or the like, a lens system having a completely symmetrical arrangement with respect to its stop is recommended. For example, Japanese Patent Post-examination Pub. No. 51-10091 discloses such a lens system having a six group and six element structure. In the copier lens system disclosed in this publication, there is obtained a sufficient level of brightness and a sufficient aperture efficiency; however, since the curvature of a radial image plane is relatively large, there is a disadvantage of incapability to maintain the flatness of an image plane to the edge portion of a field angle in use. In addition, a similar type copier lens system is disclosed in Japanese Patent Post-examination Pub. No. 61-9609, in which case, there is obtained a high contrast for high frequencies, but the coma flare is relatively large, so that there is a disadvantage of low contrast for low frequencies off the optical axis.